Numerical Simulation Of Tropical Storm Haima(2004) And Its Structure Evolution During Extratropical Transition

In this paper, Weather Research and Forecasting (WRF) model was used to simulate the extratropical transition (ET) of the tropical storm Haima. The results show that WRF can well reproduce TC intensity, track, precipitation and environment field, especially during the ET process. Haima weakened rapidly after landfall and kept barotropic structure initially. When moving northward, it approached a deep eastward-migrating midlatitude trough. As a result, to the north of the TC, the anomalous divergence at200hPa occurred on the right hand of an upper-tropospheric jet entrance. At500hPa there was a thermal trough lagging behind the though with the warm advection ahead of the trough. Meanwhile, the interaction of dry cold air mass related to the upper-level trough and warm air mass related to the TC resulted in a strong cold front. Accordingly, the warm advection and upper-tropospheric divergence in the northwestern part of the TC give rise to strong upward motion ahead of the cold front, which transited TC to a frontal cyclone. With the extratropical cyclone’s development, the upward motion and the water vapor transportation in its northwestern part were strenghened, leading to strong precipitation in the interaction region.From the analysis obove, we know the front play an important role in Haima’s extratropical transition process. As a sensitivity experiment, the tropical cyclone was filtered before the simulation using same parameters. After the removal of the typhoon, the precipitation,which was limited only frontal impacted, decreased significantly. Through the vector frontogenesis function analysis at UTC1400,it can be seen that the first item of-Fn is about the cold front frontogenesis, the second item is related to the warm front frontogenesis, and the third item can be ignored. In Fs the first two items play major roles. Fs’s values are concentrated at the junction of warm and cold front, and that makes the front do cyclonic rotation. This rotation acts only on the middle segment, so the front twists and form an occlusion. After typhoon filtered, the values of-Fn and Fs are substantially reduced, that is, the existence of the typhoon strengthens the frontogenesis, and the the interaction of Haima and the front distorted the front.Using Sawyer-Eliassen equation near the front to analyse the secondary circulation and vertical movement. Contrast the values of Q’s every item before and after the filter of the tropical cyclone. Because the corresponding of the values in Q2’s two items is better than in Q1’s itmes, Q2, that is, the geostrophic shear item plays a more important role. Before the filter, the impact of Haima on the front make Q in high-level between the front and typhoon, in low-level near Haima and ahead of the front more intense, and that induces enhanced vertical velocity. This strong upward motion is compose of two part. One is from the tropical cyclone itself, while the other is from the secondary circulation which is enhanced by the typhoon. Haima weakened rapidly after landing and then encountered a front in the mid-latitudes. The secondary circulation in this front stengthened the ascending motion of the typhoon, leading to the extratropical transition and re-development of Haima. At the same time, the existence of the typhoon strengthened the frontogenesis, and enhanced Q-item of the Sawyer-Eliassen, which was helpful for the ET process. The interaction of the typhoon and the front resulted in strong precipitation.